(a) Field of the Invention
The present invention relates to a display device, a driving apparatus of a light source for the display device, and a DC-DC converter.
(b) Description of the Related Art
Display devices used for monitors of computers and television sets include self-emitting displays such as light emitting diodes (“LEDs”), electroluminescence (“EL”) devices, vacuum fluorescent displays (“VFDs”), field emission displays (“FEDs”), and plasma panel displays (“PDPs”), and non-emitting displays such as liquid crystal displays (“LCDs”) requiring a light source.
An LCD includes two panels provided with field-generating electrodes and a liquid crystal (“LC”) layer with dielectric anisotropy interposed therebetween. The field-generating electrodes are supplied with electric voltages and generate an electric field in the LC layer, and the transmittance of light passing through the panels varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are obtained by adjusting the applied voltages.
The light may be emitted from a light source equipped in the LCD or may be natural light.
A lighting device for an LCD, i.e., a backlight unit, usually includes a plurality of fluorescent lamps such as external electrode fluorescent lamps (“EEFLs”) and cold cathode fluorescent lamps (“CCFLs”), or a plurality of light emitting diodes (“LEDs”), as light sources, which uniformly transmit the light to the entire front surface of the LC panels from the rear thereof.
When using the fluorescent lamps, characteristics of elements of the display device are deteriorated due to large power consumption and heating.
In addition, the fluorescent lamps have a stick shape that are susceptible to breakage on impact. Moreover, since temperatures of the lamps vary in accordance with positions thereof, to make luminance of the lamps vary, image quality of the LCD decreases.
However, when using LEDs, since each LED is a semiconductor device, the lifetime of the LED is long, the lighting speed of the LED is fast, and the power consumption is low. The LED also withstands impacts well and miniaturization thereof is easy.
Because of these benefits, LEDs are equipped in monitors for middle or large sized LCDs such as for computers or television sets, as well as in small sized LCDs such as in mobile telephones, for a light source.
Fluorescent lamps are activated by an alternate current (“AC”) voltage, but the LEDs are activated by a direct current (“DC”) voltage. Accordingly, for using the LEDs, the backlight unit includes a DC-DC converter for converting an AC voltage into a DC voltage and changing the DC voltage to a predetermined magnitude. The DC-DC converter may be a boost converter.
In the DC-DC converter, which includes a semiconductor switching element and a diode, a switching loss such as a turn-on switching loss or turn-off switching loss occurs in turning-on or turning-off of the switching element such that power consumption increases.
To decrease the power consumption, a soft switching method is proposed. The soft switching method includes a zero voltage switching type and a zero current switching type.
The zero voltage switching type is adapted to majority-carrier semiconductor elements having a large turn-on switching loss, and the zero current switching type is adapted to minority-carrier semiconductors having a large turn-off switching loss.
However, although the soft switching method is used, voltage and current stress of the switching element increases due to oscillation by a capacitor and an inductor such that switching availability is decreased and a conduction loss is generated. The soft switching method still has current stress due to a reverse recovery characteristic, which is a reverse current known as “a diode reverse recovery current” that flows in the diode for discharging charges charged in the diode in turning-on of the diode.